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I have a map of objects and I want to update the object mapped to a key, or create a new object and insert into the map. The update is done by a different function that takes a pointer to the object (void update(MyClass *obj))
What is the best way to "insert or update" an element in a map?
404
map. insert() will only insert elements into the container if it doesn't already contain any elements, so this will ignore the later value elements assigned to it.
C++ map update – Simple program example to update value in map. To update an existing value in the map, first we will find the value with the given key using map::find() function. If the key exists, then will update it with new value.
map insert() in C++ STL Return Value: The function returns a pair, with its member pair::first set to an iterator pointing to either the newly inserted element or to the element with an equivalent key in the map.
The operator[]
104
With something like the following snippet:
std::map<Key, Value>::iterator i = amap.find(key);
if (i == amap.end())
amap.insert(std::make_pair(key, CreateFunction()));
else
UpdateFunction(&(i->second));
If you want to measure something that might improve performance you might want to use .lower_bound() to find where an entry and use that as a hint to insert in the case where you need to insert a new object.
std::map<Key, Value>::iterator i = amap.lower_bound(key);
if (i == amap.end() || i->first != key)
amap.insert(i, std::make_pair(key, CreateFunction()));
// Might need to check and decrement i.
// Only guaranteed to be amortized constant
// time if insertion is immediately after
// the hint position.
else
UpdateFunction(&(i->second));
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